This invention relates to a lithium secondary battery in which a lithium-nickel composite oxide is used as the active material for positive electrode and more particularly to an active material for positive electrode of a lithium secondary battery that makes it possible to improve the initial capacity of the battery.
Lithium secondary batteries are compact and have a high capacity, so they have rapidly penetrated society by being used in mobile telephones, video cameras, PDAs (Personal Digital Assistants) and the like. Furthermore, recently, much research and development is being advanced with the aim of using them in automobiles such as a hybrid vehicle. Meantime, currently there is a high demand from society for batteries having high capacity, and that are safe and have excellent output characteristics.
Lithium-nickel composite oxide (hereafter referred to as LNO), which is one kind of positive electrode material used for lithium secondary batteries, has higher capacity than the lithium-cobalt oxide in current mainstream, and has advantages in that the material Ni is inexpensive than Co and it is easier to obtain, so it is expected to become the next-generation positive electrode material, and research and development is continuing for it. In a lithium secondary battery, when charging the battery, lithium dissolves into the electrolyte from the active material of the positive electrode, then passes through the separator and enters in a layer of the negative electrode, such as in the graphite layer that is capable of holding lithium. When discharging the battery, the reverse reaction occurs, and the lithium leaves the negative electrode and passes through the separator, then enters the lithium site in the lithium layer of the active material for positive electrode. Therefore, when charging and discharging a lithium secondary battery, the lithium becomes lithium ions that move back and forth between the positive and negative electrodes in this way.
When using this kind of secondary battery that uses LNO in an automobile, having high capacity is one of its important requirements for characteristics. In looking at conventional measures, for example as disclosed in Japanese Patent Publication No. Tokukai Hei 06(1994)-060887, in the case of a positive electrode having a main material of LixNiOy (where 0<x≦1.3, 1.8<y<2.2), a positive electrode material for which the X-ray diffraction intensity ratio of a specified lattice plane of the positive electrode material is regulated, and whose specific surface area according to the BET method is in the range from 0.5 m2/g to 10 m2/g, has a large discharge capacity. On the other hand, as disclosed in Japanese Patent Publication No. Tokukai Hei 10(1998)-162830, when it is premised that the sulfur content is 0.5 weight % or less in LiNiO2 powder or LiNi1-x Mx O2 powder (where 0<x≦0.4, M=at least one member selected from the group of Co, Mn, B, Al, V, P. Mg and Ti), it is preferable from the aspect of preservability that the specific surface area is 0.01 to 0.5 m2/g. In the above disclosures, however, information regarding the specific surface area of LNO for improving the initial capacity is undetermined.
Moreover, as disclosed in Japanese Patent Publication No. Tokukai 2000-30693, in order to obtain a high initial capacity, the LNO must have a higher Li site occupancy rate (97% or more). The term “Li site occupancy rate” indicates the percentage of Li ions occupying the Li site in the Li layer in the LNO crystal.
However, when the Li site occupancy rate becomes 98% or greater, it is difficult to obtain correlation with the initial capacity, so in order to develop a lithium secondary battery with even higher capacity, a new index having a correlation with the initial capacity is necessary other than the Li site occupancy rate.
In the case of the LNO described in Japanese Patent Publication No. Tokukai 2000-30693, the average particle size of the primary particles is regulated, however the average particle size of the secondary particles is not regulated.